Field of the Invention
The invention relates to a magnet valve, in particular for inlet and outlet valves of an internal combustion engine. A magnet valve generally has a ferromagnetic coil body with a winding, an armature which is displaceable in the coil body between two terminal positions to drive a valve, a spring braced between the armature and a housing, and a measuring instrument for detecting the position of the armature.
Electromagnetic drive mechanisms for inlet and outlet valves of internal combustion engines are known. In contrast to camshaft-actuated valves, magnet valves are triggered to open and close by an engine control unit as a function of the rotary position of the crank shaft. The magnet drive mechanism must be capable of exerting strong forces, especially upon the opening of an outlet valve. At all times, it must be assured that the intended terminal position of the valve is reached upon opening and closing.
In magnet valves, it is known to vary the current to the magnet winding as a function of the valve stroke as described in European Patent Application No. 0 400 389 A2. Thus during a first phase the magnet winding receives a maximum current value, which is varied at a certain frequency between an upper and a lower amplitude value. As soon as the armature is displaced and thus the air gap between the armature and the ferromagnetic winding body is reduced in size, the inductance of the winding rises. The inductance is measured and used as a measure of the course of the stroke. In a second phase the frequency of the exciter current for the winding is reduced until the termination position of the valve is reached, whereupon the coil current is switched over to a lower holding current. In other words, the known triggering of the magnet valve involves an interception current circuit, which as a function of the valve stroke triggers the winding in such a way that the valve will reliably reach the terminal position without recoil and will be held in that position. From the same European Patent Application it is also known to reduce the current to a medium value before the terminal position is reached.
The armature position of the valve can also be detected by other methods. For instance it is known to use an optical position transducer in order to detect the position of an armature that actuates a gas exchange valve, the armature being in the form of an armature disk, or to detect it with a plurality of optical position transducers as described in European Patent No. 0 493 634 B1. Using optical stroke sensors is problematic because of the risk of soiling, and magnetic stroke sensors, such as Hall sensors, furnish imprecise measurement signals, since major electromagnetic interference occurs in the vicinity of the magnet drive mechanism.